20. EXPOSURE OF AERIAL NEGATIVES 247 21. PRINTING MEDIA 252 22. PHOTOGRAPHIC CHEMICALS 257 V. METHODS OF HANDLING PLATES, FILMS AND PAPERS 23. THE DEVELOPING AND DRYING OF PLATES AND FILMS 267 24. PRINTING AND ENLARGING 279 VI. PRACTICAL PROBLEMS AND DATA 25. SPOTTING 291 26. MAP MAKING 304 27. OBLIQUE AERIAL PHOTOGRAPHY 320 28. STEREOSCOPIC AERIAL PHOTOGRAPHY 329 29. THE INTERPRETATION OF AERIAL PHOTOGRAPHS 351 30. NAVAL AERIAL PHOTOGRAPHY 368 VII. THE FUTURE OF AERIAL PHOTOGRAPHY 31. FUTURE DEVELOPMENTS IN APPARATUS AND METHODS 383 32. TECHNICAL AND PICTORIAL USES 388 33. EXPLORATION AND MAPPING 401 I INTRODUCTORY AIRPLANE PHOTOGRAPHY CHAPTER I GENERAL SURVEY =Aerial Photography from Balloons and Kites.=—Photography from the air had been developed and used to a limited extent before the Great War, but with very few exceptions the work was done from kites, from balloons, and from dirigibles. Aerial photographs of European cities had figured to a small extent in the illustration of guidebooks, and some aerial photographic maps of cities had been made, notably by the Italian dirigible balloon service. Kites had been employed with success to carry cameras for photographing such objects as active volcanoes, whose phenomena could be observed with unique advantage from the air, and whose location was usually far from balloon or dirigible facilities. As a result of this pre-war work we had achieved some knowledge of real scientific value as to photographic conditions from the air. Notable among these discoveries was the existence of a veil of haze over the landscape when seen from high altitudes, and the consequent need for sensitive emulsions of considerable contrast, and for color-sensitive plates to be used with color filters.